Implement transaction/dry-run

[emostov]

#227

Relates to paritytech/substrate#6300

This PR implements transaction/dry-run and codifies the sidecar API.

N.B. system_dryRun will not have any effects on chain state.

---

Below are some sample responses:

{
    "at": {
        "hash": "0xd46b3789ddd9d930da20fdf7a47a6fbc41e5851b325738b378351d67e65ecc4d",
        "height": "63"
    },
    "dryRunResult": {
        "resultType": "DispatchOutcome",
        "result": {
            "Error": {
                "Module": {
                    "index": "6", // balances
                    "error": "3" // InsufficientBalance
                }
            }
        }
    }
}
{
    "at": {
        "hash": "0x1de484ce19478a86d649f60c6fd79d0f2ce361555fc3706cf2cdcce38c358ebc",
        "height": "32"
    },
    "dryRunResult": {
        "resultType": "DispatchOutcome",
        "result": {
            "Ok": []
        }
    }
}

{
    "at": {
        "hash": "0xfa9aa674faacc41b0246a84bb908e8b53054dc2840ba7f6b87bcbf57e9988d5f",
        "height": "507"
    },
    "dryRunResult": {
        "resultType": "TransactionValidityError",
        "result": {
            "BadProof": null
        },
        "validityErrorType": "Invalid"
    }
}

{
    "at": {
        "hash": "0xa1c7887369ba8fc97c67a6271bf8133cbabbbc970eb95edb081f84477f7402ed",
        "height": "0"
    },
    "dryRunResult": {
        "resultType": "TransactionValidityError",
        "result": {
            "AncientBirthBlock": null
        },
        "validityErrorType": "Invalid"
    }
}

{
    "at": {
        "hash": "0x00dd123520b97da32fd88c6ed2e0b72d7686187019748d5d3bbdf10bbff0009f",
        "height": "21"
    },
    "dryRunResult": {
        "resultType": "TransactionValidityError",
        "result": {
            "Future": null
        },
        "validityErrorType": "Invalid"
    }
}

[danforbes]

Looks awesome 🎉

[danforbes]

Nice docs 🎉